Automatic white level control for a RIS

ABSTRACT

A circuit for correcting a raster input scanner or raster output scanner is described. During calibration, a calibration strip is scanned, the output of the light detector is compared to a reference voltage, and the difference is saved. This is a closed loop so that the difference value will settle at a value such that the detector output will accurately match the reference value before the final difference value is stored. In one mode, the correction is applied to one dynode of the PMT light detector, which varies the gain without varying the bandwidth of the circuit. The difference voltage is analog, but is stored in a digital memory through the use of A to D and D to A converters.

BACKGROUND OF THE INVENTION

This invention is a circuit for maintaining at a constant level thephotomultiplier output in a raster input scanner (RIS) or the lightlevel of the scanning beam in a raster out scanner (ROS), and morespecifically is a circuit for determining the amount of deviation of thelight level from the desired level during a calibrate phase, and usingthat correction value during the actual reading or writing phase ofoperation for normalizing the output.

In a raster input scanner of the type having a rotating polygon, a beamof light illuminates one polygon facet at a time to produce a flyingspot. This in turn illuminates one line of the original, and thereflected light is received by a photomultiplier to form one line ofvideo in the form of an analog voltage.

A problem in polygon systems is that the light intensity varies acrossthe line as a function of spot speed which may be slower at the centerof the line, as a function of energy distribution along the line due tochanging optical parameters (such as efficiency), and as a function ofthe angle between the original and the beam, which is off normal at theends of the line. These and other factors affect the light intensity,the corrections for which may add cost or other complications to thesystem.

One prior correction system uses a preliminary calibrate procedurecomprising the scanning of a test strip. The amount of light is measuredby the photo multiplier tube (PMT), the analog output voltage of whichis compared to a threshold. A correction voltage is determined from thiscomparison, and is stored. Later, during the regular operation of thesystem, this correction voltage is added back into the PMT output tomaintain at the output of the circuit a constant voltage for a constantlevel of original brightness. However, there are problems with thissystem. Correcting the gain of the amplifier circuit changes thebandwidth of the system, and there is no assurance that the correctionis accurate. These problems become more important in a color systemwhere variations of bandwidth and light levels may distort the colors.

Raster output scanners are similar. An intensity modulated flying spotilluminates a xerographic drum, or the like, and variations in the basicintensity of the light at the drum may create color distortions.

What is required is a system for correction of light variations thatwill not change the bandwidth of the amplifier circuits, and one thatwill make accurate corrections.

SUMMARY OF THE INVENTION

This invention accomplishes these objectives by generating a correctionvoltage during a calibrate phase which is used with a feedback circuitto create an accurate correction, and which is used to vary either thelight level, or the amplification of the PMT, to make the bandwidth ofthe system independent of the correction.

In a raster input scanner, during the calibrate phase, a test strip of aknown reflectivity, typically white or gray, is scanned by the spot, andthe output of the PMT is compared to a predetermined threshold. Thedifference is an analog voltage by which the output of the PMT must becorrected. The most common method of storing information is to store itdigitally. In this case the output of the comparator is converted todigital form and stored in RAM; then reconverted to analog for use as aninput to one dynode of the PMT.

A PMT typically has a number of dynodes, each amplifying the signalthrough secondary emission principles. The gain of each stage is afunction of the voltage difference between that node and the previousnode. Therefore, the amplification of the PMT is adjustable by varyingeither the total voltage applied to all or the single voltage applied toone dynode. Attempting to create a dynamic gain change in the PMT byvarying the total applied PMT voltage, instead of the applied voltage toa single dynode, requires a complex dynode biasing scheme. However, asonly a small gain change is required, changing the voltage of one dynodeis quite sufficient. A benefit of biasing only one dynode is that thebandwidth of the PMT does not change as a function of the PMT gain.

It may be that a correction, measured once, may not be accurate. Thissystem assures the accuracy of the correction voltage by closing theloop around the threshold detector during calibration. The corrected PMToutput is continuously coupled to the comparator and to the remainder ofthe circuit so that the final value settles to one that exactly producesthe required PMT amount.

Since the light level varies continuously over the scan, a number ofcorrections must be made. In this described embodiment, the scan isdivided into two thousand segments or sections, and each section has itsown eight bit correction factor.

After calibration, this system is used normally to scan the originalimage, and at each section of the line, the previously calculatedcorrection voltage is applied to the PMT dynode. Therefore, the PMToutput will be correctly calibrated for each section of the scan line,and any variation in PMT output will be completely the result of inputimage density.

A similar circuit can be used to calibrate a raster output scanner. Herethe light beam exposes a xerographic drum. A PMT can be used to collectlight reflected from a calibration strip on the drum. Here also, the PMToutput is compared to a threshold. The difference is converted to adigital value and stored, and a reconverted analog voltage is output.However, this time instead of controlling a PMT dynode, the voltage isused to control the original light beam intensity. This can be done, forexample, where the original light source is a light emitting diode.Then, the amount of emitted light is electrically controllable. Hereagain, the loop is closed so that the final value stored in memory willbe appropriate to produce an accurate light value.

The result of this circuit is that in either a RIS or ROS, the operationof the system compensates for light variations.

The basic difference between this invention and the prior art is thatthe prior art adds a correction based on a predetermined correctioncurve without any post-correction testing to assure that the desiredresult actually is accomplished, the desired result being a normalizedeffective optical light level. In other words, the loop is not closed.In fact, a closed loop around the process greatly improves accuracywithout the necessity of pre-determining a correction curve. Of course,pre-determined correction curves are of limited use since systemcomponents and function may vary over time. Examples of system componentchanges are, a different pointing angle from the light source, anoptical element readjusted, different performance in the light pickupdevice, and a shift in the location of the light emitting surface of thelasing diode. Another point of novelty is that the gain of the actualdetector or pickup device can be changed from inside the loop in realtime. That is, the illumination level can be changed in real time forthe needed illumination correction by scanning the calibration stripsbetween runs, or between copies.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of this invention in a raster input scanner.

FIG. 2 is a block diagram of this invention in a raster output scanner.

FIG. 3 is a block diagram of a clock generator.

FIGS. 4A-H, J-N, P-S are detailed schematic diagrams of the circuit.

FIGS. 5A-B are detailed schematic diagrams of an amplifier.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a simplified block diagram of the raster input scannercircuit. In the calibrate mode, light from a light source 10 in the formof a collimated beam is reflected from the current facet of the polygon11 onto a rotating drum. In this calibrate mode, the light is reflectedfrom a calibration strip 112 of known color which is mounted on the drumor platen just above the original. The optical pickup system 113collects the reflected light into the PMT 114, the output of which iscoupled through the "calibrate" position of switch 115 to a comparator116. The other input to the comparator 116 is the voltage 117 that thePMT should be producing from light received from the reference strip.Here the difference between the reference voltage 117 and the PMT outputis produced, the difference being the amount of error in the system.This analog voltage is converted in A to D converter 118 and stored inmemory 119 when switch 121 is in the calibrate position. This value isthen converted back at D to A converter 120 to be used to control thegain of PMT 114 by controlling the voltage at one dynode. In thealternative, the PMT output could be controlled by varying the originalamount of light from the light source, as shown by the dotted line.

Since this is a closed loop system, during each scan the various valuesin the system may take time to settle at the point where the PMT 114output exactly equals the reference voltage 117.

At the end of the calibrate sequence, the memory contains one value foreach segment of the scan line. Typically 2000 scan line segments and 12to 16 lines of data might be stored. To obtain an arithmetic average ofthe lines stored an average is taken of the stored values for everysegment from the 12 to 16 line samples. When the RIS starts to scan theactual image, switches 115 and 121 to to their "use" positions. In thiscase, the memory 119 is no longer updated, and supplies its outputthrough converter 120 to the PMT 114, the output of which is fullycorrected and useable directly by the remainder of the system.

FIG. 2 is a simplified diagram of the ROS version. Light emitting diode131 illuminates the current facet of polygon 142 which produces a flyingspot to expose one line on selenium drum 132. For calibration, anoptical pickup system 133 collects the light reflected from thecalibration strip 143 and couples it to the input of PMT 134. In the"calibrate" mode, switch 135 is closed, allowing the comparator 136 todetermine the diffrence between the PMT 134 output and the predeterminedreference voltage 137. The output is converted in A to D converter 138and, when switch 139 is in the "calibrate" position, stored in memory140. The memory 140 output is converted in D to A converter 141 into avoltage which can be used to control the amount of light emitted fromLED 131. Since this is a closed loop system, the circuit voltage willfinally settle at a point where the PMT 134 voltage equals the referencevoltage 137.

In the "use" mode, the memory 140 supplies a correction voltage throughconverter 141 to the light source 131 which illuminates the drum 132.The remainder of the circuit elements from the optical pickup 133 to theA to D converter 138 are not used.

The memory 119 of FIG. 1 and memory 140 of FIG. 2 must have addressinputs to identify the current scan line segment. There are a largenumber of different circuits that can be used to generate this addressinformation. One simple alternative is shown in FIG. 3, although acircuit designer may decide on a different configuration to suit hisapplication.

A voltage controlled oscillator 150 produces pulses. At thestart-of-scan, counter 151 starts to count these pulses and willcontinue until the end-of-scan stops it. In this embodiment the scanline is divided into two thousand segments. If the total count incounter 151 is over two thousand, counter 153 is decremented, if thecount is less than two thousand, the counter 153 is incremented.Finally, the counter 153 digital output is converted at D to A converter155 to correct the VCO 150 frequency. The result is a count that countsfrom zero to two thousand every scan. This count can be used directly asthe memory address.

The schematic drawing of the circuit is shown in FIG. 4. The output ofthe PMT is coupled in at the VIDEO INPUT, and is applied to twoisolation stages 82, 75, which prevent the following gain stages fromloading the PMT output. Two amplifiers 58, 29 provide enough gain sothat the video signal can be compared to the reference signal generatedat amplifier 43.

The reference and video signals are applied to the two input pins of theA and D converter 14 which produces an eight bit output on lines D-0 toD-7 which is a digital representation of the difference between theinputs. Thus it acts as the comparator 16 and converter 18 of FIG. 1.

During calibration, the output is applied to tri state device 13 whichinverts the signals and applies them to twenty-four inverting memorydevices 16-27, 30-41. The objective is to store an eight bit word foreach of two thousand segments for twelve lines of calibration stripvideo, the final output correction value for each segment being theaverage value for twelve lines. During the calibrate phase, twelve linesof values are stored.

A circuit detail not shown on this schematic is that the data input lineat the top of each memory device 16-27, 30-41 is the same line as thedata output line at the bottom of each device. In other words, to reducethe number of pins per device, the same pin is used for data input andoutput. Therefore, to prevent the common input data line from shortingtogether all of the data output lines, isolation devices 1-12 areinterposed between the memory devices 16-27, 30-41 and the common dataline.

Address line drivers 76-78 supply address data to the memory devices. Inthe calibrate mode, the instantaneous output of the converter 14 iscoupled through tri-state device 13, is stored in memory, but it is notused. The actual closed loop during calibration comprises tri-statedevice 28 which couples the eight bit signal to the D to A converter 73and amplifier 80 out to the PMT dynode. The circuit speed is high enoughso that all values will settle during each scan segment. At the end ofeach segment, the final digital value is stored in memory.

During use, tri-state devices 28 and 13 are turned off and tri-statedevice 57 is turned on, so that the circuit output must come frompreviously stored data in the memory. Each set of memory devices (21 and35, 20 and 34, etc.) contains two thousand values for one of twelvelines. Therefore, the commonly addressed devices are all outputting avalue for the same segment, which are averaged to determine the finalcorrection value. This averaging is done by adders 44-70 which areconnected to iteratively add two numbers and shift one bit to the rightto divide by two. This average is then coupled out through tri-statedevice 57.

Tri-state device 42 is used to supply a nominal value to the D to Aconverter 73 for use between scans when all remaining tri-state devicesare turned off. It had been found that the PMT will respond more quicklyto its correction voltage at the beginning of a scan if the voltage atthe controlled dynode is originally at a value within its normal range.

The output of amplifier 80 varies between plus one volt and minus onevolt, while a PMT dynode voltage of between negative 150 and negative275 volts is required. An amplifier of any common design can be used forthis purpose. FIG. 5 is one example. The correction voltage is amplifiedin an operational amplifier function, consisting of an amplifier 100configured as a voltage amplifier or followed by an emitter follower101, which lowers the output impedance. The result is applied to theninth dynode of the PMT.

While the invention has been described with reference to specificembodiments, it will be understood by those skilled in the art thatvarious changes will be made and equivalents may be substituted forelements thereof without departing from the true spirit and scope of theinvention. In addition, many modifications may be made without departingfrom the essential teachings of the invention.

What is claimed is:
 1. A raster input scanner comprising:means forgenerating a beam of light of a variable amount, said amount of light insaid beam being controlled by a difference signal, a rotating polygonfor producing from said beam a flying spot of light to produce scanlines, each of which is divided into a number of segments, means to bescanned by said scan lines to reflect an amount of light comprising acalibration strip of known reflectivity to be scanned during calibrationand an image area to be scanned during use, a light detector forconverting said reflected light reflected from said means to be scannedinto an electrical output, a reference voltage generator, a comparatorresponsive to said reference voltage generator and said light detectorfor calculating a difference signal between said detector output andsaid reference voltage for every segment of each scan duringcalibration, memory means for storing each difference signal for eachsegment of one scan, and for outputting said stored difference signal tosaid means for generating for corresponding segments of subsequentscans, and switching means for preventing said difference signal foreach succeeding scan from being coupled to said memory means during useso that said difference signals generated by said comparator and storedby said memory means during calibration will continue to be used duringuse.
 2. A raster input scanner of the type having means for generating abeam of light, a rotating polygon for producing from said beam a flyingspot of light to produce scan lines, each of which is divided into anumber of segments and means to be scanned by said scan lines to reflectan amount of light comprising a calibration strip of known reflectivityto be scanned during calibration and an image area to be scanned duringuse, the improvement comprising:a light detector having a variable gaincontrolled by a difference signal for converting said amount of lightreflected from said means to be scanned into an electrical output, areference voltage generator, a comparator responsive to said referencevoltage generator and said light detector for calculating a differencesignal between said detector output and said reference voltage for everysegment of each scan during calibration, memory means for storing eachdifference signal for each segment of one scan, and for outputting saiddifference signal to said light detector to vary said gain for eachcorresponding segment for subsequent scans, and switching means forpreventing said difference for each succeeding scan from being suppliedto said memory means from said comparator during use so that saiddifference signals stored during calibration will continue to be usedduring use.
 3. The scanner of claim 2 wherein said detector is a PMTcomprising a dynode for varying said gain as a function of saiddifference signal and wherein said difference signal from said memorymeans is applied to said dynode to vary said gain.
 4. The scanner ofclaim 3 wherein said difference signal received by said memory meansfrom said comparator is an analog signal, and wherein said memory meanscomprises an analog to digital converter for converting said analogdifference signal received frrom said comparator into a digital signal,a digital memory for storing said digital signal from said analog todigital converter, and a digital to analog converter for converting saiddigital signal output from said memory into an analog difference signalto be coupled to said detector.
 5. The scanner of claim 4 wherein saiddigital memory stores said digital signals from said analog to digitalconverter for all of said segments for a plurality of scan lines duringcalibration, averages said stored digital signals for each segment forsaid plurality of scan lines to produce an average digital value foreach segment, and outputs said average difference signal values to saiddetector.
 6. A raster output scanner comprising:means for generating abeam of light containing a variable amount of light, said amount beingcontrolled by a difference signal, a rotating polygon for producing fromsaid beam a flying spot of light to produce scan lines, each of which isdivided into a number of segments, means to be scanned by said scanlines to reflect an amount of light comprising a calibration strip ofknown reflectivity to be scanned during calibration and an image area tobe scanned during use, a light detector for converting said amount ofreflected light reflected from said means to be scanned into anelectrical output, a reference voltage generator, a comparatorresponsive to said reference voltage and said detector for calculating adifference signal between said detector output and said referencevoltage for every segment of each scan during calibration, memory meansfor storing each difference signal output by said comparator for eachsegment of one scan, and for outputting said difference signals storedby said memory means to said means for generating for correspondingsegments of subsequent scans, and switching means for preventing saiddifference signal for each succeeding scan from being coupled to saidmemory means from said comparator during use so that said differencesignals stored in said memory means during calibration will continue tobe used during use.
 7. The scanner of claim 6 wherein said differencesignal calculated by said comparator is an analog signal, and whereinsaid memory means comprises an analog to digital converter forconverting said analog difference signal received from said comparatorinto a digital signal, a digital memory for storing said digital signalfrom said analog to digital converter, and a digital to analog converterfor converting said digital signal stored in said memory means into ananalog difference signal to be coupled to said means for generating. 8.The scanner of claim 7 wherein said memory means stores said digitaloutput from said analog to digital converter for a plurality of scanlines during calibration, averages said digital output to produce anaverage digital output for each segment for said plurality of scanlines, and outputs said averages.